1. Field of the Invention
The invention relates generally to an apparatus for measuring the volume of an incompressible material confined within a space, and more particularly to an apparatus using an acoustical method for making such a measurement.
2. Description of the Prior Art
Various methods for measuring the amount of liquid in a storage space are currently being used. Probably the most common of these methods is an apparatus which incorporates a float designed to rest on the surface of the liquid in a tank or chamber. The position of the float (through electrical or mechanical means) is used to ascertain the total volume of the liquid in the chamber. One disadvantage with this type of apparatus is the instability of the float level resulting from movement of the tank which may be within a motorized vehicle. Movement of the tank causes the mass of liquid within to move to one side (and up one side) of the tank upon acceleration or deceleration of the vehicle. Consequently, movement of the tank results in a change in the level of the liquid at various locations within the tank, thereby altering the position of the float without a change in liquid volume. Thus, this type of prior art apparatus may require a damping or averaging meter to compensate for the effects of movement of the tank.
In a device where the level of the liquid is used to determine the volume, the correlation between the level of the liquid and volume of the liquid must be ascertained in order to provide such a measurement. The float position range must then be calibrated in order to provide an precise measurement of the volume of the liquid. However, even accurate calibration may not overcome the inaccuracies inherent in this form of of measuring device. If the float is at one side of the tank, the liquid may have a meniscus of which may prevent accurate measurement of the liquid level. This meniscus can also vary according to the type of liquid or the purity of the liquid contained in the tank. Moreover, different types of liquids typically have their different surface curvatures caused by different surface tensions. Thus, accuracy of the surface level measurement of such liquids depends to a large extent on its surface curvature and on where the float measurement is made. Thus, a different reading will be obtained depending on whether the float is positioned at the center of the liquid surface or at the edge near the side of the tank.
For measurement of a solid or a powdered substance, volumetric measurements can involve even greater difficulties. Clearly, a powdered or granular solid does not ordinarily have a level surface--particularly if this material is constantly being depleted from the tank or added to the tank. Thus, a float system of measurement is impractical with solids.
Measurements of the volume of solids and liquids having large surface irregularities have been accomplished in the prior art by ascertainment of the specific weight of the liquid or solid to be measured and the ascertainment of the weight of the chamber. The weight of the entire chamber and material contained therein is then weighed and related to the specific weight of the material in order to arrive at the volume of the material. The accuracy of this method depends to a large extent on the consistency of the specific weight value of the material and the accuracy of the weight measurement of the tank. In this regard, it must be noted that the specific weight of a material may vary substantially according to the temperature of the material. Moreover, it is not always practical to obtain an accurate weight measurement of some types of tanks. The location of such tanks may make weighing infeasible, or the tanks may be rigidly secured to another fixture. The inherent inaccuracies of float type of measurement system becomes even more acute when in a zero gravity environment such as in open space. In zero gravity environment, the shape of a liquid and a solid may be constantly changing. Pockets of liquid and gas may be interspersed throughout the storage chamber thereby preventing any accurate measurement of the surface level.
It must also be noted that in a zero gravity and zero acceleration environment, the surface level and shape of the liquid is determined by a variety of factors. Thus, a determination of the shape of the liquid and its position may be very difficult. Thus, the complexity of ascertaining the shape and location of the liquid may make prior art volume measuring devices unreliable as well as impractical.
Other prior art devices for measuring the volume of a liquid in a tank include various sensors for ascertaining the location of the surface level of the liquid. In one such prior art device, an acoustic signal is reflected from the surface of the liquid to a receiving sensor. Measurement of the time it takes to arrive at the receiving sensor makes possible a measurement of the location of the surface level of the liquid. However, as pointed out hereinabove, meniscus of the liquids, their surface curvature variations and movement of the liquid are also pertinent with this prior art system as well. Consequently, this of the float type measurement described hereinabove. method of measurement has most, if not all, of the disadvantages
Another prior art system for measuring the volume of a noncompressible liquid or solid in tank uses an acoustical means to measure the pressure of a fixed volume of gas in the tank. The pressure is inversely proportional to the gas volume for a constant quantity of gas at a constant temperature. Consequently, a measurement of the gas pressure will indirectly be a measurement of the gas volume. Moreover, a pressure change cause by a change in volume caused by the acoustic diaphragm is proportional to the volume of the gas in the tank. In a storage tank of a fixed and invariable size deducting the gas volume from the total volume of the tank will result in a measurement of the liquid or solid volume in the tank.
One prior art device uses an acoustic speaker and two transducers to measure the pressure changes of the gas. One transducer is situated within the chamber containing the material to be measured and the gas, and the other transducer is situated in a reference cavity which contains only the gas. Use of the reference cavity tends to neutralize any static pressure, as per PV/T=Gas Const; this is any variation not caused by the movement of the speaker; thus, the reference cavity tends to compensate for pressure variations due to temperature variations, mixture of another gas within the chamber static pressure changes, etc. The reference cavity is connected to the chamber by means of a small passage way. However, an obvious disadvantage of this prior art system is that a liquid to be measured within the chamber may also leak into the gage and/or reference cavity thereby altering the total volume of the material in the chamber, changing and introducing inaccuracies into the measurement. Pressure changes result from acoustic vibrations in the gas accomplished by means of the speaker diaphragm which is driven also by a transducer.
However, a primary disadvantage with this prior art system incorporating an acoustic speaker is its susceptibility to diaphragm distortion caused by splashing of the liquid within the chamber. The liquid may soak into the diaphragm causing distortion in the diaphragm's frequency of vibration. In addition, the liquid may also splash onto the diaphragm thereby adding to its weight and also consequently adding to the load onto the electromagnetic driver for the speaker; since the amplitude of vibration of the diaphragm is load-dependent, the added weight of the liquid on the diaphragm causes an undesirable alteration in the volume change (and concomitant pressure change) of the gas produced by the vibrating diaphragm. This alteration in amplitude introduces an inaccuracy in the measurement of the material volume. The liquid may also distort the shape of the diaphragm thereby making the reference cavity volume not a constant; since this prior art system bases its volume measurement on the equation: ##EQU1## where k=a constant a lack of a known constant value of V.sub.R reduces the accuracy of this measurement.
It must also be pointed out that since the acoustic speaker is operated at its resonant frequency, the liquid on the diaphragm will also alter .DELTA.P (and .DELTA.V).
The use of an acoustic speaker makes the system very sensitive and fragile; since the speaker is driven at resonant frequency, the liquid contamination and mechanical and/or acoustic vibration could make the system grossly inaccurate since the speaker must be responsive to only one sonic frequency of vibration.
The diaphragm is mounted between the reference cavity in the chamber so that displacement of the diaphragm results in a corresponding change in volume of the reference cavity as well as a change in volume of the chamber. Vibration and noise interferences are eliminated by means of a suitable pass band filter. The operating frequency of the diaphragm is preferably less than the resonance frequency of the liquid gas mixture.
It must also be noted that operating this prior art system at its resonant frequency renders the amplitude of the response to the driver frequency and/or driver energy nonlinear i.e. at resonance small incremental changes in energy input to the speaker result in disproportionately large increases in the amplitude of vibration of the speaker and consequently the change in pressure and volume of the gas. Thus, the nonlinear response of the speaker introduces gross inaccuracies in the measurement.
A disadvantage with such prior art acoustic measurement systems is the difficulty of obtaining a stable frequency of vibration of the diaphragm. A wide range of sound frequencies emitted therefrom introduces inaccuracies in the final measurements. Indeed, vibration of the diaphragm may induce vibrations in other parts of the system. Moreover, vibration problems are not completely eliminated by the pass band filters. Instead, vibration problems are extant and are likely to introduce error in the final measurement.
One of the primary disadvantages is that the reference cavity requires a passage way connecting it to the chamber; this passageway is not large enough to quickly equalize static gas pressures between the chamber and the cavity. However, enlargement of the passage way introduces leakage of liquid and/or solids into the passageway and therefore from the chamber into the cavity. Both of these occurrences introduce various inaccuracies in the measurement. It is also pertinent to note that gas diffusion from the chamber to the reference cavity or vice versa typically takes an inordinately long period of time. A primary disadvantage of this prior art acoustic measuring system is that theoretically, as the tank fills the pressure signal approaches infinity whereas the volume of the material to be measured merely approaches a certain maximum value. Conversely, as the tank approaches empties, the pressure signal approaches a certain value nonlinearily. Thus, the relationship between the tank volume and the pressure signal is a curve rather than a straight line. This nonlinear aspect of the system introduces gross inaccuracies and difficulties in measurement when the tank is either close to empty or close to full. Moreover, the complexity of the system introduces many ways in which the system may break down or compromise accuracy.